1. Field of the Invention
The present invention relates to a method of manufacturing an image forming apparatus having an image forming means and a spacer in an envelope, the spacer maintaining a space in the envelope.
2. Related Background Art
Two types of electron emitting elements are known, a hot cathode element and a cold cathode element. As the cold cathode element, a surface conduction type electron emitting element (hereinafter described as a surface conduction type emitting element), a field emission type electron emitting element (hereinafter described as FE type element), a metal/insulating layer/metal type electron emitting element (hereinafter described as MIM type element) or the like are known.
The surface conduction type emitting element is described, for example, in “Radio Eng. Electron Phys.” by M. I. Elinson, 10, 1290, (1965) and other examples to be later described are known.
The surface conduction type emitting element utilizes the phenomenon that electrons are emitted when current is flowed through a thin film having a small area formed on a substrate in parallel to the film surface. Surface conduction type emitting elements heretofore reported include an element using an SnO2 thin film by Elinson or others, an element using an Au thin film (“Thin Solid Films” by G. Dittmer, 9, 317 (1972), an element using an In2O3/SnO2 thin film (“IEEE Trans. ED Conf.”, by M. Hartwell and C. G. Fonstad, 519 (1975)), an element using a carbon thin film (“Vacuum”, by Hisashi ARAKI, et. al., Vol. 26, No. 1, 22 (1983)), and the like.
A typical example of the structure of a surface conduction type emitting element proposed by M. Hartwell is shown in the plan view of FIG. 37. In FIG. 37, reference numeral 3001 represents a substrate, and reference numeral 3004 represents a conductive thin film made of sputtered metal oxide. The conductive thin film 3004 is of an H-character shape. The conductive thin film 2004 is subject to an electric energization process called an electric energization forming process to be described later, to thereby form an electron emission area 3005. A distance L is 0.5 to 1 mm, and a width W is 0.1 mm. In FIGS. 27A and 27B, although the electron emission area 3005 is schematically shown as a rectangle at the center of the conductive thin film 3004 for the purpose of simplicity, this does not reflect the actual shape and position of the electron emission area, with high fidelity.
The electron emission area 3005 of the element proposed by M. Hartwell or other elements described above are generally formed by making the conductive thin film 3004 be subject to an electric energization process called an electric energization forming process in order to allow electrons to emit. With the electric energization, a constant d.c. voltage or a d.c. voltage rising at a very slow rate, e.g., at 1 V/min, is applied across opposite ends of the conductive film 3004 to locally destroy, deform or decompose the conductive thin film 3004 and form the electron emission area having an electrically high resistance. Cracks are formed in the conductive thin film 3004 locally destroyed, deformed or decomposed. If a proper voltage is applied to the conductive thin film 3004 after this electric energization, electrons are emitted from an area near the cracks.
As the FE type element, those elements are known which are described, for example, in “Field emission”, Advance in Electron Physics, by W. P. Dyke and W. W. Dolan, 8, 89 (1956) or in “Physical properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys. by C. A. Spindt, 47, 5248 (1976).
A typical example of the structure of an FE type element proposed by C. A. Spindt is shown in the cross sectional view of FIG. 38. In FIG. 38, reference numeral 3010 represents a substrate, reference numeral 3011 represents an emitter layer made of conductive material, reference numeral 3012 represents an emitter cone, reference numeral 3013 represents an insulating layer, and reference numeral 3014 represents a gate electrode 3014. Electrons are emitted from the tip of the emitter cone 3012 of this element through an electric field emission by applying a proper voltage between the emitter cone 3012 and gate electrode 3014.
Instead of the lamination structure shown in FIG. 38, the FE type element having a different structure is also known in which an emitter and a gate electrode are formed on a substrate generally in parallel to the substrate surface.
As an example of the MIM type element, an element described in “Operation of tunnel-emission Devices”, by C. A Mead, J. Appl. Phys., 32, 646 (1961) and other elements are known. A typical example of the structure of an MIM type element is shown in the cross sectional view of FIG. 39. In FIG. 39, reference numeral 3020 represents a substrate, reference numeral 3021 represents a lower electrode made of metal, reference numeral 3022 represents a thin insulating layer of about 100 angstroms in thickness, and reference numeral 3023 represents an upper electrode made of metal and having a thickness of about 80 to 300 angstroms. Electrons are emitted from the surface of the upper electrode 3023 of the MIM type element by applying a proper voltage between the upper electrode 3023 and lower electrode 3021.
The cold cathode elements described above can emit electrons at a temperature lower than the hot cathode element, and does not require a heater. Therefore, the structure is more simple than the hot cathode element and a fine element can be manufactured. Even if a number of elements are formed on a substrate at a high density, a problem of thermal melting of a substrate is not likely to occur. Although a response speed of a hot cathode element is low because of heating the heater, a response speed of a cold cathode element is high.
From the above reasons, applications of cold cathode elements have been studied vigorously.
For example, since a surface conduction type emitting element among cold cathode elements is simple in structure and easy to manufacture, it has the advantage that a number of elements can be formed in a large area. As disclosed in JP-A-64-31332 by the same assignee as the present assignee, a method of driving a number of elements has been studied. As the applications of surface conduction type emitting elements, an image forming apparatus for an image display device, an image recording device, a charge beam source, and the like have been studied.
As the application to an image display apparatus, an image display apparatus utilizing a combination of surface conduction type emitting elements and a fluorescent member which emits light upon application of an electron beam, has been studied as disclosed in U.S. Pat. No. 5,066,883, JP-A-2-257551, JP-A-4-28137 by the same assignee as the present assignee. An image display apparatus utilizing a combination of surface conduction type emitting elements and a fluorescent member is expected to have more excellent characteristics than a conventional image display apparatus of other types. For example, as compared to a recently prevailing liquid crystal display apparatus, the image display apparatus of this type does not require back light because of self light emission and has a broad angle of view.
A method of driving a number of FE type elements is disclosed in U.S. Pat. No. 4,904,895 by the same assignee as the present assignee. An example of the application of FE type elements to an image display apparatus is a flat panel type display apparatus reported by R. Meyer in “Recent Development on Microtips Display st LETI”, Tech. Digest of 4th int. Vacuum Microelectronics Conf., Nagahama, pp. 6–9 (1991).
An example of the application of a number of MIM type elements to an image display apparatus is disclosed in JP-A-3-55738 by the same assignee as the present assignee.
Of image forming apparatuses utilizing the above-described electron emitting elements, a flat panel type display apparatus having a thin depth requires less space and is light in weight. Therefore, the flat panel type display apparatus has drawn attention as a substitute for a CRT type display apparatus.
FIG. 40 is a perspective view showing an example of a display panel portion of a flat panel type image display apparatus. A portion of the panel is broken in order to shown the internal structure.
In FIG. 40, reference numeral 3115 represents a rear plate, reference numeral 3116 represents a side wall, and reference numeral 3117 represents a face plate. The rear plate 3115, side wall 3116 and face plate 3117 constitute an envelope (air-tight envelope) which maintains the inside of the display panel vacuum.
A substrate 3111 is fixed to the rear plate 3115. N×M cold cathode elements 3112 are formed on the substrate. N and M are positive integers of 2 or larger and are properly set in accordance with a target number of display pixels. The N×M cold cathode elements 3112 are wired as shown in FIG. 40 by M row direction wiring lines 3113 and N column direction wiring lines 3114. A structure made of the substrate 3111, cold cathode elements 3112, row direction wiring lines 3113, and column direction wiring lines 3114 is called a multi electron beam source. At each cross area of the row direction wiring line 3113 and column direction wiring line 3114, an insulating layer (not shown) is formed between the lines to provide electrical insulation.
A fluorescent film 3118 made of fluorescent material is formed on the bottom surface of the face place 3117. The fluorescent materials of red (R), green (G) and blue (B) colors of three primary colors are divisionally coated to form the fluorescent film 3118. Black color material (not shown) is coated between the color fluorescent materials of the fluorescent film 3118. A metal back 3119 made of Al or the like is formed on the fluorescent film 3110 on the side of the rear plate 3115.
Dx1 to Dxm, Dy1 to Dyn, and Hv are electrical connection terminals of an air-tight structure for electrically connecting the display panel to an unrepresented electric circuit. Dx1 to Dxm are electrically connected to the row direction wiring lines 3113 of the multi electron beam source, Dy1 to Dyn are electrically connected to the column direction wiring lines 3114 of the multi electron beam source, and Hv is electrically connected to the metal back 3119.
The inside of the air-tight envelope is maintained at a vacuum of about 10−6 Torr. As the display area of the image display apparatus becomes large, a pressure difference between the inside of the air-tight envelope and the outside thereof becomes large. It is therefore necessary to provide means for preventing the rear plate 3115 and face plate 3117 from being deformed or destroyed. If the rear plate 3115 and face plate 3117 are made thick, not only the weight of the image display apparatus increases, but also an image distortion increases when viewed obliquely and a parallax may occur. In the example shown in FIG. 40, structural support members (called a spacer or rib) 3120 made of relatively thin glass plates are mounted in order to be resistant to the atmospheric pressure. The distance between the substrate 3111 with the multi electron beam source and the face plate 3117 with the fluorescent film 3118 is maintained usually sub-mm or several mm, and the inside of the air-tight envelope is maintained highly vacuum as described earlier.
As a voltage is applied to each cold cathode element 3112 via the terminals Ds1 to Dxm and Dy1 to Dyn of the image display apparatus using the above-described display panel, electrons are emitted from each cold cathode element 3112. At the same time, a high voltage of several hundreds V to several kV is applied via the terminal Hv to the metal back 3119 to accelerate the emitted electrons and make them collide with the inner surface of the face plate 3117. The fluorescent materials of each color constituting the fluorescent film 3118 emit light and an image can be displayed.
A spacer having a space maintaining function sufficient for maintaining the space in the air-tight envelope of the image display apparatus described above has been desired, and also a method of efficiently forming the spacer has been desired.